1. Field of Art
The disclosure relates to forming a gallium-nitride (GaN) layer on a substrate using an atomic layer deposition (ALD) process or a pseudo-ALD process.
2. Description of the Related Art
Gallium nitride (GaN) is a very hard material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV affords it special properties for applications in optoelectronic, high-power and high-frequency devices. Hence, Gallium-nitride-based semiconductor has found its application in electro-optical components such as light emitting diodes (LED), laser diodes and detectors as well as high performance electronic components such as transistor operable with high power in high frequency region at a high temperature. Generally, GaN semiconductor is fabricated by heteroepitaxially growing a GaN layer on expensive sapphire or silicon-carbide (SiC) substrates.
A GaN layer can be epitaxially grown on a substrate, for example, using metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Generally, the combination of Triethygallium (TEGa) and ammonia (NH3) are used in MOCVD to epitaxially grow the GaN layer. However, such MOCVD is carried out in a high temperature where diffusion between the GaN layer and a buffer layer below the GaN layer may occur. That is, molecules of the lower buffer layer may diffuse into the upper GaN layer and Ga molecules in the upper layer may diffuse into the lower buffer layer and N molecules as well.
Such diffusion may cause, among other phenomena, distorting of a lattice structure in the lower buffer layer, distorting of a lattice structure in the GaN layer initially formed on the substrate. This increases internal stress in GaN layer and/or the buffer layer as well as increasing discontinuity between the GaN layer and the lower buffer layer. As a result, it is difficult to obtain a good quality GaN epitaxial layer using the MOCVD. Moreover, the high temperature (e.g., over 1,000° C.) during the MOCVD process may cause depreciation of nitrogen from ammonia and disassociation of carbon molecules from TeGa. The disassociated hydrocarbon gas may flow into the region of the substrate being processed, disrupting the homogeneity and stoichiometry of the layer.
In order to growing a GaN layer using MBE, a high degree of vacuum (e.g., 10−10 Torr or lower) is needed. MBE also results in low deposition rate and high production cost. Further MBE is generally inadequate for depositing a GaN layer on a large substrate or producing silicon substrates with the GaN layers in mass quantities.